The present invention relates to an objective optical system which is installed in a device employing multiple types of light beams having different wavelengths, such as an optical information recording/reproducing device for recording information to and/or reproducing information from multiple types of optical discs differing in recording density.
There exist various standards of optical discs (CD, DVD, etc.) differing in recording density, protective layer thickness, etc. Meanwhile, new-standard optical discs (HD DVD (High-Definition DVD), BD (Blu-ray Disc), etc.), having still higher recording density than DVD, are being brought into practical use in recent years to realize still higher information storage capacity. The protective layer thickness of such a new-standard optical disc is substantially equal to or less than that of DVD. In consideration of user convenience with such optical discs according to multiple standards, the optical information recording/reproducing devices (more specifically, objective optical systems installed in the devices) of recent years are required to have compatibility with the above three types of optical discs. Incidentally, in this specification, the “optical information recording/reproducing devices” include devices for both information reproducing and information recording, devices exclusively for information reproducing, and devices exclusively for information recording. The above “compatibility” means that the optical information recording/reproducing device ensures the information reproducing and/or information recording with no need of component replacement even when the optical disc being used is switched.
In order to provide an optical information recording/reproducing device having the compatibility with optical discs of multiple standards, the device has to be configured to be capable of forming a beam spot suitable for a particular recording density of an optical disc being used, by changing a NA (Numerical Aperture) of an objective optical system used for information reproducing/registering, while also correcting spherical aberration which varies depending on the protective layer thickness changed by switching between optical discs of different standards. Since the diameter of the beam spot can generally be made smaller as the wavelength of the beam gets shorter, multiple laser beams having different wavelengths are selectively used by the optical information recording/reproducing device depending on the recording density of the optical disc being used. For example, for DVDs, a laser beam with a wavelength of approximately 660 nm (so-called red laser light) shorter than approximately 790 nm for CDs is used. For the aforementioned new-standard optical discs, a laser beam with a wavelength still shorter than that for DVDs (e.g. so-called “blue laser” around 408 nm) is used in order to deal with the extra-high recording density.
In recent years, a technology for suitably converging a light beam onto a record surface of an optical disc being used by providing an optical element having an annular zone structure for an objective optical system in an optical information recording/reproducing device has been brought into practical use. More specifically, the annular zone structure formed on a surface of the optical element is configured to have a plurality of annular zones divided by minute steps. Through the function of the annular zone structure, each of the multiple types of light beams having different wavelengths is suitably converged onto the record surface of each of the optical discs of different standards.
Japanese Patent Provisional Publication No. 2006-185576A (hereafter, referred to as JP2006-185576A) discloses an example of an optical information recording/reproducing device having compatibility with optical discs of multiple standards.
In order to perform information recording or information reproducing for an optical disc having a high recording density, the optical information recording/reproducing device is required to form a beam spot having a higher intensity than that for existing optical discs. For this reason, the annular zone structure is designed such that the use efficiency of the blue-laser used for information recording/reproducing for an optical disc having a high recording density is set at a high level. The use efficiency of light means a contribution ratio of the amount of laser light emitted by a light source with respect to the information recording/reproducing. The fact that the use efficiency of light is high means that each optical component in the optical information recording/reproducing device has a high transmissivity.
However, when an annular zone structure designed to give an optical path length difference substantially equal to an odd multiple of the wavelength of the blue laser and to achieve the high use efficiency for the blue laser is used in an objective optical system of the optical information recording/reproducing device, use efficiency of light (i.e., use efficiency of near-infrared laser light) in the information recording/reproducing for CD inevitably decreases.
In order to solve such a problem, an objective optical system disclosed in Japanese Patent Provisional Publication No. 2006-12394A (hereafter, referred to as JP2006-12394A) is configured to have an optical element formed by cementing two types of optical components made of different materials with respect to each other. A diffraction structure is formed on a cementing surface between the two types of components so that the optical element enhances the use efficiency of near-infrared laser light while keeping the diffraction efficiency of blue laser light at a high level.
However, the objective optical system disclosed in JP2006-12394A has a drawback that a process for cementing the two types of optical components has to be additionally processed in the manufacturing process of the objective optical system. In addition, the diffraction structure has to be formed on the cementing surface with extremely high accuracy. Therefore, the manufacturing cost of the objective optical system is high.
Japanese Patent Provisional Publication No. 2007-122828A (hereafter, referred to as JP2007-122828A) discloses an optical pick-up device configured to have a diffraction grating formed by laminating at least two types of elements having different degrees of dispersion together so that high diffraction efficiency can be maintained for both of the blue laser and the near-infrared laser. JP2007-122828A also discloses an optical pick-up device provided with an optical element having a single diffraction surface designed to appropriately select, for each of the blue laser and the near-infrared laser, the diffraction order at which the diffraction efficiency is maximized.
However, in order to manufacture the optical pick-up device having the above described optical pick-up, the manufacturing has to be performed with an extremely high degree of accuracy. Therefore, the manufacturing cost of the optical pick-up disclosed in JP2007-122828A is very high. The optical element having the single diffraction surface disclosed in JP2007-122828A is configured to produce even-order diffracted light for the blue laser. In this case, it becomes difficult to correct the relative spherical aberration caused by switching between an optical disc requiring the blue laser and an optical disc requiring the near-infrared laser.